U.S. Pat. No. 4,435,905 granted to J. B. Bryan on Mar. 12, 1984 described a telescoping magnetic ball bar test gage that serves to measure the accuracy of machine tools, robots and non-disengageable servo drives of the type that cannot be clutched out which primarily include numerically controlled machines. Essentially, this patent teaches a gage with a telescoping bar having a pair of gage balls mounted on each end and a parallel reed flexure unit and linear variable differential transformer assembly for executing a prescribed circular trajectory. Magnetic socket knuckle assemblies cooperate with the gage balls to locate the telescoping unit between a point on a remote work table and the machine being measured. While this instrument is particularly efficacious for making precise measurements of changes in the radial direction which can be used for gaging the accuracy of the machine being measured, the instrument is limited not only by the radial direction but also by the fact that it can only make measurements within the radius extending over its range of travel. However, it cannot determine the coordinates of the measured point. Of significance is that it is limited to measuring in a radial direction and lacks the capability of making two (2) and three (3) dimensional measurements.
Moreover, since the ball bar of the instrument described in the U.S. Pat. No. 4,435,905, supra, has finite displacement bounded substantially adjacent the circumference of its circular trajectory it falls within a class of apparatus that will only indicate when the machine being tested will not meet a predetermined specification. Also the metes and bounds of the specification is confined to the circular trajectory. This is in contrast with this invention that not only can determine when a machine meets the standards as prescribed above, but it will indicate whether or not the machine being tested will pass the tolerances defined in a given specification and which specification goes beyond the circular trajectory described in the prior art.
This invention constitutes an improvement over the apparatus described in the U.S. Pat. No. 4,435,905, supra, by not only extending the range of measurement to a three dimensional spatial location but also to extend the range to permit measurements that are not restricted to a circular or limited trajectory. This invention contemplates utilizing one or more telescoping fixtures each of which are integrated with an interferometer mounted between two spherical gage fittings carried at each end of the telescoping fixture. The output of the laser ball bar (LBB) gage of this invention can be used with the technique of trilateration to determine the coordinates of a point in space. It also contemplates a method of converting differential measurements produced by the interferometer into absolute lengths.